New example to generate AST from scratch (#507)

* feat: new example generate ast from scratch

* fix: max cols and function naming

* fix: import path

examples/construct_ast_from_scratch.py

@@ -0,0 +1,53 @@
+# -----------------------------------------------------------------
+# pycparser: construct_ast_from_scratch.py
+#
+# Tiny example of writing an AST from scratch to C code.
+#
+# Andre Ribeiro [https://github.com/Andree37]
+# License: BSD
+# -----------------------------------------------------------------
+import sys
+
+# This is not required if you've installed pycparser into
+# your site-packages/ with setup.py
+sys.path.extend(['.', '..'])
+
+from pycparser import c_ast, c_generator
+
+
+# target C code:
+# int main() {
+#     return 0;
+# }
+
+
+def empty_main_function_ast():
+    constant_zero = c_ast.Constant(type='int', value='0')
+    return_node = c_ast.Return(expr=constant_zero)
+    compound_node = c_ast.Compound(block_items=[return_node])
+    type_decl_node = c_ast.TypeDecl(declname='main', quals=[],
+                                    type=c_ast.IdentifierType(names=['int']),
+                                    align=[])
+    func_decl_node = c_ast.FuncDecl(args=c_ast.ParamList([]),
+                                    type=type_decl_node)
+    func_def_node = c_ast.Decl(name='main', quals=[], storage=[], funcspec=[],
+                               type=func_decl_node, init=None,
+                               bitsize=None, align=[])
+    main_func_node = c_ast.FuncDef(decl=func_def_node, param_decls=None,
+                                   body=compound_node)
+
+    return main_func_node
+
+
+def generate_c_code(my_ast):
+    generator = c_generator.CGenerator()
+    return generator.visit(my_ast)
+
+
+if __name__ == '__main__':
+    main_function_ast = empty_main_function_ast()
+    print("|----------------------------------------|")
+    main_function_ast.show(offset=2)
+    print("|----------------------------------------|")
+    main_c_code = generate_c_code(main_function_ast)
+    print("C code: \n%s" % main_c_code)